1. Field of the Invention
This invention relates to a method of preparing .omega.-lactams, in particular caprolactam, with improved yield and purity.
It is known that caprolactam can be prepared by reacting cycloaliphatic compounds with a nitrosating agent in the presence of a dehydrating agent.
U.S. Pat. No. 3,356,675 and Italian Patent Application 27018 A/79 disclose preparing caprolactams from cycloaliphatic compounds, including hexahydrobenzoic acid, by reacting them with nitrosyl acid sulphate in concentrated sulphuric acid or oleum, and utilizing oleum as the dehydrating agent.
Italian Patent Application 27018 A/79 claims a process for preparing .omega.-lactams containing 5 to 14 carbons in a multi-stage reactor, implemented by racting a cycloaliphatic acid having the general formula ##STR2## where n varies from 3 to 13, and/or the corresponding anhydrides, with a nitrosating agent in the presence of a dehydrating agent.
According to the prior art, moisture is to be removed completely from the lactamization reaction medium to avoid the need to decrease the reaction rate and selectively below an acceptable minimum. It is, in fact, known that the reaction rate and selectivity can only be increased through the use of an appropriate dehydrating agent.
Examples of cycloaliphatic acids which can be lactamized include hexahydrobenzoic acid, cyclododecanecarboxylic acid, etc. Hexahydrobenzoic acid, also referred to as cyclohexanecarboxylic acid, being preferred.
As the nitrosating agent, any of the agents specified in U.S. Pat. No. 3,356,675 may be used.
Nitrosyl sulphuric acid is preferred for the production of caprolactam. Known dehydrating agents are: SO.sub.3, chlorosulphonic acid, anhydride of chlorosulphonic acid, phosphoric anhydride, or even the very anhydride of hexahydrobenzoic acid, in which case, in calculating the molar ratios, one mole anhydride of hexahydrobenzoic acid is the equivalent of one mole SO.sub.3 ; thus, for simplicity, reference will be made hereinafter to SO.sub.3 -equivalent moles, to specify either one mole SO.sub.3 or one mole of some other anhydride serving the same anhydrifying function.
The reaction temperature can vary in the range of 30.degree. to 100.degree. C., and in particular where hexahydrobenzoic acid is used, the reaction temperature would be preferably in the 60.degree. to 90.degree. C. range.
The reaction may be conducted in either discontinuous, semicontinuous, or continuous stirred reactors.
A continuous multi-stage reactor would be preferred with hexahydrobenzoic acid, anyhow.
Thermal regulation of the reaction (which is markedly exothermic) may be effected by using, in the reaction system, a particular inert thermostating solvent which can remove reaction heat by absorption as evaporation heat, thereby constancy of the reaction temperature is achieved by boiling the thermostating solvent.
Examples of thermostating solvents are: cyclohexane, n-eptane, n-hexane, n-pentane, chlorinated or fluorinated hydrocarbons, nitrocompounds, etc.
It is known to conduct the lactamization reaction in conformity with the following sequence of process steps:
1. Premixing step: wherein mixing is carried out at a low temperature, i.e. in the range of 0.degree. to 40.degree. C., of the hexahydrobenzoic acid with the dehydrating agent (e.g. oleum at a concentration in the 30% to 37% range); during this step, there occurs formation of a mixed anhydride which, in the instance of caprolactam, comprises cyclohexanecarboxysulphonic anhydride.
2. First reaction step: wherein there occurs the lactamization reaction between the mixed anhydride and the nitrosating agent (e.g. nitrosylsulphuric acid in oleum). This step takes place according to the continuous method within a multistage reactor where the nitrosating agent is fed into each of the stages at a temperature in the 30.degree. C. to 100.degree. C. range. This method affords excellent control of the reaction exothermicity through the above-mentioned thermostating solvent.
3. Second reaction step: wherein the reaction is completed and no nitrosating agent is fed; this step takes place in a multistage system, in series and/or within an external reactor (post-reactor) at a temperature in the 30.degree. C. to 100.degree. C. range.
4. Unreacted hexahydrobenzoic acid reclaiming step: wherein low temperature hydrolysis (approximately 20.degree. C.+30.degree. C.) is carried out of the reaction mass along with the extraction of the hexahydrobenzoic acid by the same thermostating solvent.
The control parameters of this reaction are:
Molar ratios of load and temperature SO.sub.3 S.sub.tot NOAEB(*). FNT (*)hexahydrobenzoic acid.
By operating in accordance with these prior methods, on an industrial scale, the following lactamization outputs are to be obtained, when expressed as kmoles of caprolactam yielded per kmole hexahydrobenzoic acid (.eta.AEB). EQU .eta..sub.AEB =0.85+0.90
Furthermore, the byproducts that forms when operating in accordance with prior methods are of four types:
i. strongly acidic byproducts to be obtained by sulphonation, in the reaction conditions, of the alpha-carbon in the carboxylic group of hexahydrobenzoic acid; cyclohexane-carboxylic acid is formed which represents the largest percent amount of all the byproducts formed; PA0 ii. byproducts with a weak acid function (carboxylic acids), such as .epsilon.-amino-capronic-N-hexahydrobenzoyl, .epsilon.-amino-valeric-N-hexahydrobenzoyl, etc.; PA0 iii. neutral byproducts, such as lactones, amides, nitriles, etc.; PA0 iv. high polarity byproducts which are, therefore, readily soluble in water; this byproduct family also represents an important reasons for not achieving higher outputs.